This invention relates to a method and structure for the encapsulation of microelectronic circuits by injection molding, while minimizing the distortion of bonding wire geometry.
The flow of a plastic melt into a mold cavity during the molding or encapsulation of a plastic IC package exerts forces sufficiently high as to displace or deform the bond wires. Generally referred to in the industry as "wire sweep", this is the predominant cause of defects in the molding of IC packages. Wire sweep is defined as the linear deviation of the bond wire from a straight line drawn between the bond pad and the lead frame as projected on the plane of the leadframe.
The ensuing wire deformation can cause adjacent bond wires to come into contact with each other, or they may collapse onto an electrically active part of the chip, both of which cause the device to be electrically shorted.
High density circuits such as 16 Mega Bit memory devices use the newly developed lead-on-chip (LOC) packaging technology wherein the bond pads are located at the centre of the chip. This poses added difficulties since the bond wires need to span over the power buss bars to be bonded to the lead fingers. There are risks of shorting between adjacent wires, and shorting to the power buss bars.
The mechanics of wire sweep and the often interrelated factors affecting the viscous fluid flow are complex. However, some of the factors found to influence wire sweep can be summarized as follows:
1. Mold design--a runner and gating system in which filling is simultaneous in all cavities and allows the individual fill rate to be reduced, thereby avoiding cavity filling rush in the first and the last cavities. It is the fluid velocity within the cavity that deflects the wires. PA1 2. Plastic flow imbalance between the 2 cavity halves--flow in one half is faster than the other, causing the leadframe to tilt thereby subjecting each wire to both tensile and bending forces. PA1 3. Air vent design--this needs to be strategically located such that it is not blocked by the flow of the plastic. A blockage will entrap air which, when compressed by the packing action, can induce wire deflection. PA1 4. Viscosity of the plastic melt--the forces exerted on the bond wires increase with the viscosity of the flowing plastic thereby increasing the degree of wire sweep. PA1 5. Wire and wire material--for a given force a larger wire diameter is naturally more resilient to deformation but this would increase material cost, especially if gold wire is used. A small addition of beryllium to gold increases the elastic property of the wire in the annealled condition such that it increases the elastic energy under its stress-strain cure. This has the effect of minimizing the overall wire sweep since a larger portion of wire sweep due to elastic deformation is now recoverable. PA1 6. Molding parameters--preform preheat time/temperature, mold temperature, transfer pressure and transfer time have very important impact on wire sweep:
Underheated preforms will be too viscous initially and affect the sweep of the initial units. Overheated preforms will set up too quickly and affect the sweep of the final units and interfere with mold packing. PA2 Mold temperature has effects similar to preheat conditions. Low temperatures result in high viscosity, and high temperatures in rapid cure. PA2 Transfer pressure tends to affect only the final stages of filling and mold packing: the greater the pressure the more severe the sweep. PA2 A short transfer time (high transfer rate) will cause severe sweep; a long transfer time (low transfer rate) will permit a higher viscosity before the end of the cycle and hence less severe sweep. PA2 The ram height setting above the preform has a significant impact on the sweep since a high setting can effectively change a long apparent transfer time into a very short actual material transfer time.
Hi Density LOC Devices
Large, high density chips (in the region of 330.times.660 mils) together with the sub-micron wafer fabrication technology has meant that the properties required of the encapsulating compound differ somewhat from those of the previous generation devices. A better package integrity and thermal matching of the compound with the other assembly components require, amongst other things, increments in the filler content. This can improve significantly both its mechanical strength and the coefficient of thermal expansion which in turn reduces the package stress. A complete description of such compositions, and their use, appears in U.S. Pat. No. 4,632,798 incorporated herein by reference.
Although increments in the filler content tend to improve strength and reduce stress, they also increase the viscosity of the compound, leading to potential wire sweep problems as the following results appear to show.
These results are indicative of the need to reduce wire sweep in this new package: the manufacturing specification of 15% sweep (for conventional packages) was exceeded in many cases. For wire bonds that spanned over the buss bars, there is a potential risk of these wires collapsing onto these buss bars, causing an electrical short. This risk is naturally increased as the degree of wire sweep is increased, since a swept wire is more likely to collapse than one which is not. There is thus a need to reduce or eliminate this potential problem.
With all the bond pads aligned in a straight line at the centre of the chip, these devices are now found to be more amenable to a bond configuration that is less susceptible to wire sweep. Both lead frame design and bond pad locations at the periphery of the chip, as in the conventional packages, have been the main constraints for the most desirable wire bond configuration.